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© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009 UK: +44 (0)1353 648 888 US: +1 973 983 9588 www.prismsound.com Same.

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Presentation on theme: "© Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd. 2009 UK: +44 (0)1353 648 888 US: +1 973 983 9588 www.prismsound.com Same."— Presentation transcript:

1 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Same Speaker, Different Data? Loudspeaker design and analysis tools Presented at ALMA Asia Symposium 2010

2 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Introductions Simon Woollard Prism Sound Prism Sound: –Established 1987 (23 years) –Founders Ian Dennis and Graham Boswell designed worlds first digital audio console at AMS Neve –Ian Dennis co-wrote AES-ABU digital interface standard –Professional audio logging / recording / editing solutions –Audio test and measurement equipment Simon Woollard: –BSc (Hons) Audio Technology, University of Salford, UK –Worked as loudspeaker design engineer (NXT, Mission) –Now Applications Engineer for Prism Sounds Test and Measurement Division

3 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Agenda Introduction Typical Measurement Setup Discussion: Sources of Inconsistency Recap Conclusion

4 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Part 1 Introduction Typical Measurement Setup Discussion: Sources of Inconsistency Recap Conclusion

5 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Introduction: Measurement Inconsistency THE PROBLEM - Same Loudspeaker: Different data!

6 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: We ship samples & employees across continents to investigate Introduction: Measurement Inconsistency

7 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: We ship samples & employees across continents to investigate Introduction: Measurement Inconsistency Measurement inconsistency costs time & money!

8 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Part 2 Introduction Typical Measurement Setup Discussion: Sources of Inconsistency Recap Conclusion

9 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Typical Measurement Setup Typical Loudspeaker Measurements: –On-axis frequency response –Off-axis frequency response (polar plot) –Power response –Distortion / non-linearities –Impedance –Thiele-Small parameters Today we will discuss on-axis frequency response; Perhaps the most common loudspeaker measurement

10 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Typical Measurement Setup Typical Loudspeaker Measurements: –On-axis frequency response –Off-axis frequency response (polar plot) –Power response –Distortion / non-linearities –Impedance response –Thiele-Small parameters Today we will discuss on-axis frequency response; Perhaps the most common loudspeaker measurement

11 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Typical frequency response measurement setup: Typical Measurement Setup

12 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System Typical frequency response measurement setup: 1 Typical Measurement Setup

13 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier Typical frequency response measurement setup: 1 2 Typical Measurement Setup

14 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT Typical frequency response measurement setup: Typical Measurement Setup

15 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle Typical frequency response measurement setup: Typical Measurement Setup

16 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone Typical frequency response measurement setup: Typical Measurement Setup

17 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber Typical frequency response measurement setup: Typical Measurement Setup

18 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Typical frequency response measurement setup: 1)Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber POTENTIAL SOURCES OF MEASUREMENT INCONSISTENCY Typical Measurement Setup

19 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Part 3 Introduction Typical Measurement Setup Discussion: Sources of Inconsistency Recap Conclusion

20 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber Sources of Inconsistency: Test and Measurement System

21 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Calibration of test system (2 main types): 1) Dedicated Hardware: –Usually calibrated by manufacturer –Typical accuracy ±0.05dB (absolute), ±0.05dB flatness –Should include amplitude ranging to ensure maximum noise / distortion performance at low signal amplitude 2) Generic Soundcard: –Requires calibration by operator, or errors could be ~dBs! –Noise and distortion of soundcard may affect accuracy at low amplitude (frequency response affected less) Sources of Inconsistency: Test and Measurement System

22 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Test and Measurement System For Todays Examples dScope Series III Hardware dScope Software USB Cyrus III G.R.A.S. 46AE / 26CA IEC Baffle

23 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Possible stimuli for frequency response measurement: Stepped sine (sweep) Broad band noise Maximum length sequence (MLS) Continuous sweep (chirp) Multi-tone Sources of Inconsistency: Test and Measurement System

24 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Impulse response (using broad band stimulus) is widely used: Sources of Inconsistency: Test and Measurement System

25 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Impulse response (using broad band stimulus) is widely used: Sources of Inconsistency: Test and Measurement System

26 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Impulse response (using broad band stimulus) is widely used: Sources of Inconsistency: Test and Measurement System

27 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Impulse response methods require a broad band stimulus Stepped sine sweep cannot remove room reflections Stepped sine sweep Log swept sine + Impulse response Sources of Inconsistency: Test and Measurement System

28 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Window Function: Prism 7 Prism 5 Gaussian Blackman-Harris Hamming Hann Blackman Triangular Rectangular Sources of Inconsistency: Test and Measurement System Choice of window function affects frequency response:

29 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Choice of window function affects frequency response: Window Function: Prism 7 Prism 5 Gaussian Blackman-Harris Hamming Hann Blackman Triangular Rectangular Sources of Inconsistency: Test and Measurement System

30 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Length of window function affects LF cut-off: Sources of Inconsistency: Test and Measurement System t

31 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Length of window function affects LF cut-off: HF also affected by reflections in non-anechoic measurements Window Length: t = 5ms t = 10ms t = 15ms t = 20ms t = 25ms t = 30ms Sources of Inconsistency: Test and Measurement System Shorter window

32 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Low frequency cut-off is given by: f = 1 / t f = lowest valid frequency in measurement (Hz) t = length of time domain window (seconds) Sources of Inconsistency: Test and Measurement System

33 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Test and Measurement System Stimulus Amplitude: Voice coil driving force is non-linear WRT transducer excursion At high excursion, efficiency will be lower (e.g. low frequency) mm

34 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: At high excursion, efficiency will be lower (e.g. low frequency): Stimulus Amplitude: 15V 7.5V 3.75V 500mV 250mV 100mV 50mV Sources of Inconsistency: Test and Measurement System

35 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Start position of window can affect low frequency response: Correct Window Position Sources of Inconsistency: Test and Measurement System

36 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Correct Window Position Window Starts too early Start position of window can affect low frequency response: LF attenuation due to early start of window function Sources of Inconsistency: Test and Measurement System

37 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber Sources of Inconsistency: Power Amplifier

38 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Output / cable impedance: Output impedance and cable impedance should be minimised, and ideally calibrated in the test and measurement system Sources of Inconsistency: Power Amplifier

39 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Assume a loudspeaker impedance as follows: Assume amplifier output impedance + cable impedance is 0.5Ω; We see error if we measure without calibration Loudspeaker impedance Frequency Response Measurement Error Sources of Inconsistency: Power Amplifier

40 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber Sources of Inconsistency: Baffle

41 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Why do We need a Baffle? Positive voltage > outward cone motionAir compresses in front of diaphragm (positive pressure)Air expands behind diaphragm (negative pressure)Acoustic short circuit at low frequencyNo significant acoustic radiationUsing a baffle restores air pressure in front of diaphragm

42 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: If the wavelength is long compared to the baffle dimensions, radiation is omnidirectional (4π) If the wavelength is short compared to the baffle dimensions, radiation is in the forward direction only (2π) : SPL is 6dB greater Sources of Inconsistency: Baffle Low Frequency High Frequency

43 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: IEC / AES standards for baffle size; varies with transducer size IEC / AES standard baffle dimensions for transducers up to 8 diameter Sources of Inconsistency: Baffle

44 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Baffle should be large compared to wavelengths of interest If baffle is too small, we see low SPL at low frequencies: Sources of Inconsistency: Baffle Baffle Size: 1650 x 1450mm (IEC) 650 x 550mm

45 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Ensure a good air seal between transducer and baffle If air seal is poor, we see errors at low / mid frequencies: Sources of Inconsistency: Baffle Air Seal: Airtight Poor Seal

46 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber Sources of Inconsistency: Microphone

47 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Use a dedicated test and measurement microphone Calibrate the microphone using manufacturers data Calibration data for G.R.A.S. 46AF mic S/N Sources of Inconsistency: Microphone

48 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: For highest precision, use a pistonphone or calibrator G.R.A.S. pistonphone type 42AA G.R.A.S. calibrator type 42AB Sources of Inconsistency: Microphone

49 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Correct for actual atmospheric pressure Sources of Inconsistency: Microphone

50 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Use the correct microphone capsule! Typically a choice of 2: Pressure microphone: For Measuring SPL at a boundary Mic does not modify pressure field Free-field microphone: For measuring SPL in free space Mic modifies pressure field, so is acoustically equalised to correct error Sources of Inconsistency: Microphone

51 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Free-field response of a ½ pressure mic capsule: Image from Cirrus Research Sources of Inconsistency: Microphone

52 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Using wrong capsule type gives error at high frequency: Sources of Inconsistency: Microphone Free-field measurement: Free-field mic capsule Pressure mic capsule

53 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Use of non-instrument grade mic, and / or generic calibration data will be less accurate: Sources of Inconsistency: Microphone Mic: G.R.A.S. Type 46CA (calibrated) Brand x mic, no calibration data supplied

54 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Be careful if you are not measuring at a distance of 1m Response is affected at close mic distances: Sources of Inconsistency: Microphone We can measure at any distance and normalise to the equivalent 1m sensitivity… x

55 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Be careful if you are not measuring at a distance of 1m Response is affected at close mic distances: Sources of Inconsistency: Microphone Microphone Distance: x = 0mm x = 50mm x = 100mm x = 200mm x = 400mm x = 800mm

56 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Comb filtering caused by path differences from cone to speaker: Sources of Inconsistency: Microphone x y If y – x = n*wavelength: CONSTRUCTIVE interference If y – x = (n - 0.5)*wavelength: DESTRUCTIVE interference (where n is integer)

57 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Comb filtering caused by path differences from cone to speaker: x and y approximately equal in far field; comb filtering is minimised Sources of Inconsistency: Microphone x y

58 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: )Test System 2)Amplifier 3)EUT 4)Baffle 5)Microphone 6)Room / Chamber Sources of Inconsistency: Room / Chamber

59 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Room / Chamber Reflections from room boundaries cause comb filtering

60 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Room / Chamber Reflections from room boundaries cause comb filtering x

61 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Room / Chamber Reflections from room boundaries cause comb filtering y y x

62 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Room / Chamber Reflections from room boundaries cause comb filtering x If 2y – x = n*wavelength: CONSTRUCTIVE interference If 2y – x = (n – 0.5)*wavelength: DESTRUCTIVE interference (where n is integer) Path difference = 2y - x y y

63 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Room / Chamber Reflections from room boundaries cause comb filtering Early Reflection No Early Reflection

64 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Sources of Inconsistency: Room / Chamber Early Reflection No Early Reflection Reflections from room boundaries cause comb filtering

65 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Reflections can be removed from an impulse response But we lose low frequency information from our measurement Sources of Inconsistency: Room / Chamber

66 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Reflections can be removed from an impulse response But we lose low frequency information from our measurement Sources of Inconsistency: Room / Chamber Window Length: 5ms 10ms 15ms 20ms 25ms 30ms

67 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Use a large room Sources of Inconsistency: Room / Chamber

68 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Move the microphone & speaker away from room boundaries Sources of Inconsistency: Room / Chamber

69 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Move the microphone & speaker away from room boundaries Sources of Inconsistency: Room / Chamber

70 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Use absorption if possible (anechoic chamber) Sources of Inconsistency: Room / Chamber

71 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Small chambers are very sensitive to mic position and reflections Sources of Inconsistency: Room / Chamber

72 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Small chambers are very sensitive to mic position and reflections –Make the microphone mounting secure and accurate –Use non-simple ratios between chamber x, y, and z dimensions –If possible, use non-parallel chamber walls –Use high specification acoustic absorption –Use high stiffness, high damping, high mass chamber walls to prevent panel resonances and noise transmission –Ensure that multiple chambers are built to identical specifications Sources of Inconsistency: Room / Chamber

73 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Part 4 Introduction Typical Measurement Setup Discussion: Sources of Inconsistency Recap Conclusion

74 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Earlier we saw 2 measurements of the same loudspeaker: We have now identified the major causes of the discrepancies Recap

75 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Causes of frequency response measurement inconsistency: –Test System: Calibration Stimulus type / measurement method Stimulus amplitude Window function (for impulse response) Window length Window start time –Power amplifier: Output impedance / cable impedance Calibration for output impedance –Baffle: Baffle size - IEC / AES recommend standard baffle sizes Air seal between transducer and baffle Recap

76 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Causes of frequency response measurement inconsistency: –Microphone: Use a dedicated T&M mic (omni-directional) Capsule type: pressure or free-field Calibration for sensitivity, frequency response and mic pre gain Correction for atmospheric pressure Correction for any gain in ear couplers Distance from mic to loudspeaker –Room / Chamber: Nearby boundaries Room dimensions Windowing (if used) Position of mic Recap

77 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Part 5 Introduction Typical Measurement Setup Discussion: Sources of Inconsistency Recap Conclusion

78 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: –Sources of ERRORS should be MINIMISED / calibrated, e.g.: Cable and amplifier impedance Microphone frequency response Environmental noise –Measurement VARIABLES should be STATED with measurement data, e.g.: Stimulus type Stimulus amplitude Microphone distance Type of window function Baffle dimensions Room dimensions Conclusion

79 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: On-axis frequency response re 2.83V, 1m Conclusion Suggested datasheet format: Brand XYZ, model ADCD, S/N

80 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Conclusion By minimising ERRORS and documenting VARIABLES, we can all save time and money!

81 © Copyright Prism Media Products Ltd. 2008© Copyright Prism Media Products Ltd UK: +44 (0) US: Further Considerations We could also talk about measurement inconsistencies in: –EUT; atmospheric temperature, humidity, voice coil temperature due to test cycle duration etc –Impedance, TSPs, distortion measurements etc –Electrical measurements – power amplifiers, crossovers etc –Headphones / ear couplers / HATSs But we are out of time! Perhaps a topic for ALMA 2011…


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